Page 209 - NS-2 Textbook

204 NAUTICAL SCIENCES

toward the South Pole. The African plate crashed into the Earth's mantle like giant chlmks of ice, moved by the
Eurasian plate south of Europe and pushed up the Pyre- churnings in the interior. Where these plates come to-
nees mountain range between Spain and France, the Alps gether, Earth and its inhabitants experience the awesome
in France and Switzerland, and the Apennines of Italy. On energy of earthquakes and volcanoes. Seismograplls,
the other side of the globe, the Pacific plates pushed up the modern instnunents that measure the intensity of earth-
Andes in South America, the Sierras along the West Coast quakes, have helped to locate the boundaries of the
of North America, and the islands of Japan. plates, called fault lines. Also along these bOlmdaries,
In time the continents gradually took the places on mOlUltains rise and fall and volcanic islands push up
the globe that are familiar to us today. The major ocean from the sea. The energy released in the explosion of
basins and numerous seas-once a single ocean mass a nuclear bomb is small compared with these huge geo-
"vith one giant continent-now provide the vital sea lines logie forces.
of cOlnmunication and conlmerce between the -widely Earthquakes. The great earthquake belts that lie along
separated continents. The globe as we know it today is the plate margins are extremely important to sailors and
the result of a geologic process that has taken billions of people v\Tho live on seacoasts and in harbors. Volcanoes
years and continues even nnw. have created ne,v islands and island chains-the Ha\vai-
ian Islands, some Aleutian and Japanese islands, and is-
THE EARTH'S CRUST TODAY lands in the Caribbean and Mediterranean Seas, among
others. In the United States, the entire West Coast is in an
The evolutionary process just discussed created a global
earthquake "belt." The best-known feature of this belt is
jigsaw puzzle of segments known as geological plates. The the San Andreas Fault, which runs through the center of
plates drift over the uppermost, semimolten layer of California and close to San Francisco. In fact, some geol-
ogists predict that all of Baja California and much of the
present state of California may someday break away
from the North American continent and drift toward
Alaska, arriving there in about 50 million years!
But not all such catastrophes will happen in the dis-
tant future. In fact, many earthquakes occur daily. Tokyo,
Japan, for example, often experiences two to three
tremors each day. Forhmately, few are ever felt by people,
though sensitive seismographs do record several hun-
dred of them each year. In 1902 MOlmt Pelee, a volcano
near St. Pierre on the Caribbean island of Martinique,
erupted with an earthquake and superheated gases that
killed 30,000 people within seconds. In 1906 San Francisco
was almost totally destroyed by a large quake on the San
Andreas Fault. Within the past ten years, devastating
quakes have killed thousands of people in Italy, Iran, Pak-
istan, Turkey, Greece, Guatemala, Nicaragua, Mexico, and
the former Soviet Union. Another less serious but widely
reported quake disrupted the baseball World Series in Oc-
tober 1989 in San Francisco, causing much damage and at
least sixty-three confirmed deaths. The largest disaster of
all time from a single earthquake occurred in 1976 in
Tangshan, China, when almost 700,000 people were re-
ported to have been killed.
Tsul1ami. When an earthquake or volcanic explosion
happens near or lmder the sea, ocean waves radiate from
it in ever-widening circles. TIl€f€ may be little movement
detected on the open sea, but as these waves reach shallmv
,vaters along coastlines, the waves slow dmvn and pile up
The progression of continental drift. (A) The original supercontinent in huge crests, sometimes more than 100 feet high. These
Pangaea, 200 million years ago. (8) The world 135 million years ago. huge ·waves are called tSll1w11li, a Japanese ,vard that
Pangaea has split into Laurasia, to the north, and Gondwanaland, means "surging ,valls of water." 111€Se fantastic walls of
to the south. (e) Our world today. India has collided with Eurasia,
and Australia has split from Antarctica; North and South America water can race across the deep oceans at jet-plane speeds
have joined in Central America. of 450 miles per hour (mph) but then slow to 25-30 mph